Summaries leave out detail or examples that may distract the reader from the most important information, and they simplify complex arguments, grammar and vocabulary. Used correctly, summarizing and paraphrasing can save time, increase understanding, and give authority and credibility to your work.
You will want to paraphrase or summarize when the wording of the source is less important than the meaning of the source. The paraphrase and summary allow you to maintain continuity of style in your paper and show your mastery of source material. A paraphrase may be preferred to a summary because paraphrases are more detailed and specific.